The study only lasted six weeks, and researchers call the evidence preliminary. But they say it's the first large study to show that slashing nicotine, perhaps below an addiction threshold, is safe and leads to less smoking.

The Food and Drug Administration was given the power in 2009 to mandate lower nicotine levels if it would help public health, but has not yet done so.

Leading cause of heart disease and cancer

"This, I think, provides support" for lowering nicotine, said one study leader, Dr Neal Benowitz of the University of California, San Francisco.

"What our study shows is that it's feasible," and that people won't smoke more regular cigarettes to compensate, he said.

Results are in the New England Journal of Medicine. The FDA and the National Institute on Drug Abuse paid for the study. Two study leaders have advised companies that make smoking cessation aids, and two testified in tobacco lawsuits.

Smoking is a leading cause of heart disease and cancer. Tar and other substances inhaled through smoking make cigarettes deadly, but the nicotine in tobacco is what makes them addictive.

Some earlier work suggests they might not be if nicotine was limited to roughly 0.7 milligrams per gram of tobacco. Most cigarettes contain around 15.8 milligrams per gram of tobacco.

There are no low-nicotine cigarettes on the market; the government made special ones with several lower nicotine levels to test.

"We wanted to see how much lower it would need to be to see that effect," where dependence did not happen or was diminished, said another study leader, Dr Eric Donny, a University of Pittsburgh psychologist.

For the study, about 800 people who smoked five or more cigarettes a day and had no interest in quitting were assigned to smoke either their usual brand or an experimental type with nicotine ranging from a low of 0.4 milligram to 15.8 milligrams, the level in most cigarettes.

The cigarettes were provided for free, and no one except people assigned to keep smoking their regular brand knew how much nicotine any of their smokes contained.

Fewer symptoms of nicotine dependence

The low-nicotine users were a little more likely to smoke some regular cigarettes in addition to the ones provided in the study, yet the overall number of cigarettes and nicotine levels were lower in those groups.

During the last week of the study, those given cigarettes with low amounts of nicotine 2.4, 1.3 or 0.4 milligrams per gram of tobacco were averaging 15 or 16 smokes a day. That compares to 21 or 22 cigarettes for those given the 15.8-milligram cigarettes or their usual brand.

One low-nicotine level 5.2 milligrams did not cause any change in number of cigarettes smoked from smokers' usual brand.

The study was not intended or designed to get smokers to quit. But twice as many in the low-nicotine group than those smoking standard-strength cigarettes 35 percent versus 17 percent said they had tried in the month after the study ended.

A longer study is under way to see whether a gradual or abrupt shift to low-nicotine cigarettes is best.

Dr Michael Fiore and Timothy Baker, tobacco researchers from the University of Wisconsin, Madison, wrote in a commentary in the journal that the study shows the potential for a policy to cut nicotine that "could help to end the devastating health consequences" of smoking.

The FDA declined to comment on the study, but the director of its Centre for Tobacco Products, Mitch Zeller, said in a statement that "though all tobacco products are potentially harmful and potentially addictive, different categories of tobacco products may have the potential for varying effects on public health."

Two large cigarette makers R.J. Reynolds Tobacco Co., which sells Camel and other brands, and Altria Group Inc., which owns Philip Morris USA declined to comment.

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